1. Field of the Invention
The present invention relates to a sheet stacking apparatus and an image forming apparatus.
2. Description of the Related Art
In recent years, thanks to technological advances, an image forming apparatus has become capable of forming images at high speed. Together with the increase in image forming speed, sheet discharging speed from the image forming apparatus has also increased. As a result, demand for a high-volume sheet stacking apparatus capable of precisely aligning the sheets is increasing.
Japanese Patent Application Laid-Open No. 2006-124052, for example, discusses a sheet stacking apparatus which includes a pressing member that presses a sheet to a sheet stacking tray so that the sheet can be more speedily discharged onto the sheet stacking tray.
FIG. 10 illustrates a configuration of a conventional sheet stacking apparatus 100 which enables high-volume output. The sheet stacking apparatus is attached to a conveying belt 508 that rotates clockwise and includes a gripper 503. The gripper 503 rotates together with the conveying belt 508 to convey a sheet while holding a leading edge of the sheet. Further, the sheet stacking apparatus includes a leading edge pressing member 506 and a trailing edge pressing member 507 configured to press down a leading edge and a trailing edge of a sheet.
In the sheet stacking apparatus having such a configuration, a sheet discharged from an image forming apparatus (not shown) is received by an inlet roller 501 and then a leading edge of the sheet is passed on to the gripper 503 by a conveyance roller 502. Then, the conveying belt 508 rotates, and the gripper 503 moves together with the conveying belt 508 while holding the leading edge of the sheet. In this way, the sheet is conveyed along the upper portion of a sheet stacking tray 505.
When the leading edge of the sheet abuts against a leading edge stopper 504, the gripper 503 releases the sheet so that the sheet is discharged onto the sheet stacking tray 505. In this manner, a predetermined number of sheets are stacked. Every time a sheet is stacked, an alignment member (not shown) performs a jogging process in a direction perpendicular to the sheet conveying direction (hereinafter referred to as width direction) so that an alignment of the sheets is improved.
When sheets are stacked at high speed, possibility of a sheet jam is increased, which occurs when a sheet interferes with a trailing edge of a preceding sheet stacked on the sheet stacking portion 505. Therefore, during sheet stacking, the leading edge pressing member 506 and the trailing edge pressing member 507 press down a leading edge and a trailing edge of a sheet against the sheet stacking tray so that the sheet reaches the sheet stacking tray 505 more quickly.
In other words, when sheets are stacked at high speed, the leading edge pressing member 506 and the trailing edge pressing member 507 press a leading edge and a trailing edge of a sheet against the sheet stacking tray 505 at the time the sheet is discharged to the sheet stacking tray 505 so that the sheet is out of the way of the next sheet.
However, in such a conventional sheet stacking apparatus, when a sheet is pressed to the sheet stacking tray by the leading edge pressing member 506 and the trailing edge pressing member 507, a path on which the sheet takes from a release step to landing is not fixed. Therefore, accuracy of stacking position is considerably poor.
Especially when priority is given to pressing by the trailing edge pressing member 507 to increase stacking speed, the sheet can be pressed in a state that the sheet leans against the stacking wall Y. In this case, not only damage is given to the sheet but also accuracy of stacking deteriorates.
Also, in a case where the leading edge pressing member 506 presses a leading edge portion of a sheet, the leading edge pressing member 506 can be configured to wait at a position 506′ and then rotate counterclockwise around a center of rotation X. In this case, a pressing force acts also in the right direction as shown in FIG. 10.
Thus, in a case when a leading edge of a sheet is pressed by the leading edge pressing member 506, not only a force acts on the sheet stacking stray 505 but also acts in the right direction in FIG. 10. Accordingly, the leading edge of the sheet is curled. As a result, stacking accuracy is decreased and an undesired curl will be formed on the sheet.
Consequently, if a sheet discharged onto the sheet stacking tray 505 is simply pressed by the leading edge pressing member 506 and the trailing edge pressing member 507, stable stacking with high accuracy is not achieved.